This invention relates to photometric apparatus and finds particular application in electronic colour photometers, especially those constructed for use as colour analysers, colour temperature meters and graphic arts camera photometers.
In such instruments the range of intensity measurement required is commonly some four or more decades and the percentage accuracy is desirably constant. For this reason it is the common practice to construct such instruments to read logarithmic intensity increments on a linear scale and, for this reason, such a photometer is commonly termed a log photometer.
Log photometers can be constructed using, as a light sensor, a photomultiplier operated in the "constant anode current mode" or a photomultiplier operated at constant EHT with or without logarithmic conversion or by using other photo cells, for example selenium or silicon operating into an infinite impedance circuit. Such instruments are well known and will not be described further except in so far as it is necessary to explain their use in the apparatus of the present invention.
The present invention is concerned with the processing of information obtained by making certain measurements with the photometer and with ensuring that the results obtained are valid. It is convenient at this stage to consider more particularly the basic construction and use of a colour analyser. Such an instrument consists of a log photometer, usually having a photomultiplier sensor. Used with the sensor is a light source and a filter system having three switchable primary colour filters, each associated with a corresponding read-out zero setting control, and, in addition, an "open filter" (i.e. an aperture or filter transmitting white light) also with zero control. To use this instrument in the printing of a photographic negative, the instrument is first employed to assess a known negative in a photographic printing set-up which will yield a neutral balanced correctly exposed print. For this purpose, the photomultiplier is put into the exposing area or a selected part thereof in the printing set-up and, with the various filters successively put between the light source and the sensor, each zero control is in turn set so that the instrument read-out, which may be a galvanometer or digital volt meter or other form of indicator, indicates balance. When a production negative, that is a negative to be printed, is inserted into the printing set-up, it is then only necessary to adjust the primary colour light intensities to the levels set up in the colour analyser and then print the negative by exposing it for the same time as used for the reference negative in order to achieve a satisfactory print. In rudimentary instruments, the common practice is to use a lens iris as the means of adjustment of the total light. More advanced instruments incorporate an electrical control, frequently termed a "master control" or "electronic iris" which is used to compensate for overall density deviation of the production negative from the programmed reference to bring the read-out to balance point on one colour and to permit the operator to bring the remaining two colours back to balance by the use of filters. This master control must only operate on the colour channels.
It will be seen that the colour analyser is used essentially for comparing light intensities. A similar requirement arises in a colour temperature meter. As is well known, the spectral energy distribution, that is to say the relationship between energy and wavelength, differs for different light sources. As the colour temperature (that is the temperature of a perfectly radiating black body when emitting light matching the source under test) rises, the relative blue content increases and the red content decreases. Colour temperature measurement is essentially concerned therefore with the comparison of the red and blue content of a light source. A tricolour temperature meter has to measure the relative intensities of the red and blue components compared with the green.
A graphic arts camera photometer is used to evaluate the maximum and minimum densities of the copy to provide, for exposure control, information about the minimum density and the total density range.